The present invention relates to a piezoceramic material suitable for use in piezoelectric sensors such as pressure sensor, acceleration sensor, knock sensor, yaw rate sensor, gyro sensor and shock sensor and other piezoelectric devices, a piezoelectric element using the piezoceramic material and a non-resonance knock sensor using the piezoelectric element.
Various piezoelectric sensors have widely been used in the areas of electronics and mechatronics. Each of the piezoelectric sensors includes a piezoelectric element equipped with a sintered piezoceramic body (bulk) and at least one pair of element electrodes so as to convert a mechanical stress applied thereto to an electricity or voltage by the piezoelectric effect of the piezoceramic body and generate an electrical signal based on the converted electricity or voltage. The sensitivity (output voltage) of the piezoelectric sensor varies as the piezoelectric characteristics of the piezoceramic body change with the ambient temperature of the operating environment. When the temperature of the piezoelectric sensor changes during operation or changes with the ambient temperature of the operating environment, there occurs a thermal stress in the piezoelectric element due to the difference in thermal expansion between the piezoceramic body and the element electrodes or other adjacent sensor parts. There also occurs a voltage in the piezoelectric sensor by the pyroelectric effect of the piezoceramic body in response to the temperature change. These introduce noise into the output of the piezoelectric sensor and thus lead to variations in the sensitivity of the piezoelectric sensor. Further, the piezoelectric characteristics of the piezoceramic body may deteriorate to cause a decrease in sensor output under the weight applied to the piezoelectric element. For the above reasons, the operating temperature of the piezoelectric sensor is generally set to about −40° C. to 170° C. It is however desired that the piezoelectric element exhibit no variations in temperature characteristics over a wider temperature range in view of the fact that the piezoelectric sensors are used under severe conditions in e.g. automotive engines.
In order to achieve such a temperature-stable piezoelectric element, Japanese Patent No. 2789374, Japanese Patent No. 2964265 and Japanese Patent No. 2957002 disclose PZT (lead zirconate titanate) piezoceramic materials doped with Sn, Nb and Sb so as to obtain improvements in thermal stability by the addition of Sn and to enable low-temperature sintering and softening (i.e. improvements in piezoelectric characteristics with large crystal distortion) by the addition of Nb and Sb.